Keppel Bay is a complex coastal sedimentary system dominated by strong tidal currents and episodic flood discharge from the adjacent Fitzroy River. The sedimentary environments of the bay have been assessed using surface sediment samples, sediment cores and geophysical techniques. The distribution of surface sediments in Keppel Bay reflects the three primary benthic environments: muddy tidal channels, shallow muddy inner bay, and the sandy outer bay where relict sediments are reworked by tidal currents and storms. Very little modern river sediment is deposited on the middle and outer continental shelf. River sediment that is not trapped in the estuary and Keppel Bay appears to be transported onshore and northwards.
Keppel Bay is a semi-enclosed shallow coastal embayment. It is bounded to the north by Great Keppel Island and to the south by Curtis Island. It is a drowned landscape, featuring numerous rocky islands and prominent coastal headlands. Keppel Bay has a macro tidal range of 5 m that generates strong tidal currents, and receives highly episodic flood discharge from the Fitzroy River.
Figure 1. Satellite image of the Fitzroy River lower floodplain, estuary and Keppel Bay. The different areas studied are outlined in red and include the beach ridges to the west of Keppel Bay.
Sediment cores, surface grab samples, acoustic sub-bottom profiles, and multibeam (or swath) sonar mapping were used to characterise the bed of Keppel Bay. Cores were used to indicate the rate at which sediment accumulates in the different areas and changes in the benthic environment over time. Sub bottom profiles, multibeam sonar data, and bathymetry data were used to delineate the morphology of the seabed and help characterise benthic environments.
The bathymetry of Keppel Bay suggests a very diverse range of sea bed environments. Near the mouth of the Fitzroy Estuary there are a series of deep muddy tidal channels and sand bars. Some of the channels appear to be infilling, while others display erosional features. Inner Keppel Bay is relatively flat and primarily accumulates river-derived muddy sediment that blankets the underlying old floodplain. The surface sediments in the outer bay include very little modern river sediment. Ancient river channels are evident in the sub-bottom profiles and show that in some areas, especially in the outer bay, there is minimal or no sediment deposition. Data from the sonar surveys at several sites show large sand dunes on the seabed that are being moved onshore.
Figure 2. Detailed bathymetric map of the Fitzroy Estuary and Keppel Bay. A high-resolution multibeam image of sediment dunes on Centre Bank is shown below (red box on map indicates location).
Figure 3. Stratigraphic cross section and sub-bottom profiles for Keppel Bay. Inner Keppel Bay is shallow and flat, ancient channel structures are overlain by 2-3 m of modern coastal sediment (A). In the outer bay, east of the line of bedrock islands, ancient channels are not covered in modern sediments, or are partially infilled because this area is starved of modern coastal sediment (B). In some areas of the outer bay the sediment is being eroded and reworked and a series of marine sediment dunes are transporting shelf sediment into the southeast of the bay (Figure 2).
Offshore from Keppel Bay the shelf is approximately 100 km wide. The edge of the shelf is lined by the reefs of the Capricorn Bunker group, which are the southernmost reefs of the Great Barrier Reef. During the last ice age, when sea level was lower, much of this shelf was exposed. The detailed bathymetry map shows the ancient channel of the Fitzroy River which meandered across the flat coastal plain before entering the Capricorn Channel. The ancient Fitzroy River channel is only partially infilled because very little modern river derived sediment is reaching the outer continental shelf.
Figure 4. Detailed bathymetry east of Keppel Bay shows the path of the ancient Fitzroy River that meandered across the shelf.
Figure 5. Sub-bottom profile of the ancient Fitzroy River channel. The profile shows that the channel is partially infilled from the south by marine sediments. The old well-bedded muddy river sediments partially infill the base of the channel.